This invention relates generally to the art of fluid level sensors, such as might be used in gasoline or oil reservoirs, and more particularly concerns sensors which produce an output dependent upon the refractive index of the medium in which a sensing element portion is immersed.
In most fluid reservoirs, there is sensing apparatus for determining the existing level of the fluid. Typical examples of such reservoirs are fuel reservoirs in aircraft and in vehicles such as automobiles, or storage tank facilities of various kinds. In many of these applications, it is important that the sensing apparatus be electrically passive, and for that reason, sensors have been developed which use the properties of light to determine fluid level. In such sensors, the light entering the sensing element is either refracted into the medium in which it is immersed, or reflected back to a readout apparatus, depending on the critical angle of the element/medium combination and the angle of the entering light. The angle of the entering light is controlled so that for a medium of interest, i.e. gasoline, it is greater than the critical angle, resulting in reflection, while for another medium, i.e. air, it is less than the critical angle, resulting in refraction. The light output, which will thus vary depending on the medium in which the element is located, thus indicates whether or not the element is in or out of the particular medium of interest.
A wide variety of sensors based on this basic optical principle are known, although such sensors have not been previously widely used, due to several significant disadvantages. First, the outputs of many of the sensors are not reliable and are sometimes difficult to interpret. Also, the performance of such sensors often quickly degrade with continued use. Such sensors often require frequent repair and/or replacement, and are usually quite complex, both in operation and configuration, which increases the chances of failure, and/or incorrect results. A complex configuration, for instance, permits buildup of contaminants on the surface of the sensor, leading to a decrease in accuracy.
Accordingly, a general object of the present invention is to provide a sensor which is capable of overcoming one or more of the disadvantages of the prior art noted above.
It is another object of the present invention to provide such a sensor which is capable of accurately determining the boundary between two mediums, having different indexes of refraction, such as gasoline and air.
It is an additional object of the present invention to provide such a sensor which is configured so that substantially all of the light entering a sensing element portion of the sensor is reflected to an exit point when the sensor is immersed in a first medium, and so that substantially all of the light entering the sensing element portion is refracted out of the sensing element into the surrounding medium when the sensor is immersed in a second medium having a different index of refraction than the first medium.
It is a further object of the present invention to provide such a sensor which is so configured that contaminants do not build up on its surface during use.
It is an additional object of the present invention to provide such a sensor which includes a plurality of sensing elements formed into an array.
It is a still further object of the present invention to provide such a sensor which has an output which is substantially independent of the angle of the light entering the sensing element.